The replicative history and replicative potential of human naive and memory T cells, critical parameters of lymphocyte biology, were analyzed. Telomeres are unique terminal chromosomal structures which shorten with cell division in vitro and with increased age in vivo for human somatic cells. We assessed telomere length as a measure of the in vivo replicative history of naive and memory human T cells, and found that telomeric terminal restriction fragments were 1.4 0.1 kb longer in CD4+ naive T cells than in memory cells from the same donors, a relationship that was constant over a wide range of donor age. This suggests that the differentiation of memory cells from naive precursors occurs with substantial clonal expansion that is similar over a wide age range. The in vitro replicative capacity of naive cells was 128-fold greater than that of memory cells from the same donors. Human CD4+ naive and memory cells thus differ in in vivo replicative history as reflected in telomeric length as well as in their residual replicative capacity. These relationships may be significant for pathologies such as HIV infection, in which CD4+ T cell generation may be compromised, and for therapeutic interventions mediated by cells whose in vivo expansion is essential for therapeutic effect. Telomerase, a ribonucleoprotein that is capable of synthesizing telomeric repeats, is expressed in germline and malignant cells and is absent in most normal human somatic cells. The selective expression of telomerase has thus been proposed to be a basis for the immortality of the germline and of malignant cells. When telomerase activity was analyzed in normal human T lymphocytes, it was found that telomerase is expressed at a high level in thymocytes, at an intermediate level in tonsil T cells, and at a low to undetectable level in peripheral blood T cells. Moreover, telomerase activity was highly inducible in peripheral T lymphocytes by activation through CD3 and CD28 (anti-CD3/CD28). Telomerase may thus play a permissive role in T cell development and in determining the capacity of lymphoid cells for clonal expansion.